In recent years, a so-called wireless power supply technique of performing power transfer without contact by electromagnetic coupling between coils has been widely used. As methods for wirelessly transferring electric power, various methods have been conventionally proposed. In recent years, a wireless power supply technique by a so-called “electromagnetic inductive method” has attracted attention. The electromagnetic inductive method uses the principle of electromagnetic induction that when an electric current is passed to one of two adjacent coils, an electromotive force is generated in the other adjacent coil with a magnetic flux which is generated as a medium, and products loaded with wireless power supply systems by the electromagnetic induction method have also started to be put into practice.
In the wireless power supply system by the electromagnetic induction method, a one-way communication from a power receiving side to a power transmission side is performed. A load modulation circuit is connected to an antenna coil on the power receiving side, and the load of the load modulation circuit is varied, whereby the load capacity on the power receiving side is changed. Thereby, the amplitude of the coil voltage on the transmission side is changed, and binary ASK (amplitude shift keying) is realized. Generally in a load modulation circuit using a capacitor, a transistor is used as a switching element, and accumulation and discharge of electric charges to and from the capacitor are controlled, whereby the total load capacity on the power receiving side is changed.
In the wireless power supply system by the electromagnetic induction method, a one-way communication from a power receiving side to a power transmission side is performed. A load modulation circuit is connected to an antenna coil on the power receiving side, and the load of the load modulation circuit is varied, whereby the load capacity on the power receiving side is changed. Thereby, the amplitude of the coil voltage on the transmission side is changed, and binary ASK (amplitude shift keying) is realized. Generally in a load modulation circuit using a capacitor, a transistor is used as a switching element, and accumulation and discharge of electric charges to and from the capacitor are controlled, whereby the total load capacity on the power receiving side is changed.
The transistor used as the switching element is generally incorporated into the same chip as a peripheral circuit by using a C-DMOS (complementary and doublediffused MOS) process, and therefore is configured by an LDMOS (laterally diffused MOS) with high withstand voltage. In such a configuration, when the capacitor which is used for load modulation is in a state where electric charges are accumulated in the capacitor, the transistor is in an off state, and a drain voltage drops by a forward voltage or more from a reference voltage (GND), a parasitic diode formed between the substrate and the well diffusion region of the same transistor becomes a discharge path of the capacitor.
However, when the transistor is formed by using a C-DMOS (complementary and doublediffused MOS) process, the same transistor and the peripheral circuit are not completely separated electrically, and therefore, a parasitic bipolar transistor with a near diffusion region in contact with the substrate (the diffusion region of the same conductive type as that of the well diffusion region) as a collector is operated by the discharge current by the parasitic diode. Accordingly, the problem arises, that the possibility of the peripheral circuit using the near diffusion region to be the supply source of a collector current operating erroneously becomes high. Further, when a high potential is applied to the diffusion region, the problem arises, that power consumption by the collector current of the parasitic bipolar transistor becomes so large that the heating value increases.